fix(rules): resistance is a +1 resistance bonus on fort/ref/will saves for 1 minute - SRD spells/resistance.md grants the subject a +1 resistance bonus on saving throws for 1 minute (Will negates, harmless); the resistance bonus type already exists in the stacking model (effects.py BonusType), so the buff is expressible as a BuffEffect and was only deferred because _find_buff_spell returns the FIRST BuffEffect in known-spell order and cantrips cost no slots - encoding it would have made default_policy cast Resistance every round (for Tammara it would preempt Divine Favor and never kill anything); resistance.json now carries a BuffEffect (+1 resistance on fort/ref/will, 10 rounds = 1 minute) and _find_buff_spell skips level-0 spells (cantrips have no spell slot in the engine's budget - spell_slots_left has no level-0 key - and the policy deliberately never auto-casts them, a cantrip buff would be cast every round degrading the policy), so Resistance cannot preempt a leveled buff (Divine Favor) in the known-spell order; leveled buffs keep the historical first-match behavior (if the first one's slots are exhausted the buff branch gives up - the golden pins that); the resistance bonus type does not stack with itself (resolve_modifiers keeps the higher value); +9 tests (test_resistance.py: buff shape, cast on a willing ally consumes no d20 and applies without a save roll, the bonus rides save rolls, no self-stacking, expiry at 10 rounds, _find_buff_spell skips the slot-less cantrip / returns None without a leveled buff, _choose_spell_action proposes Divine Favor and never Resistance); golden untouched; 1714 tests green

data/spells/resistance.json

@@ -7,6 +7,16 @@
   "harmless": true,
   "base_dc": 10,
   "cast_time": "standard",
-  "effects": [],
+  "effects": [
-  "description": "SRD spells/resistance.md: a cantrip - the subject gains a +1 resistance bonus on saving throws for 1 minute. Will negates (harmless). Deferred: the resistance bonus type exists in the stacking model, so the buff is expressible - but _find_buff_spell returns the FIRST BuffEffect spell in known-spell order and cantrips cost no slots, so encoding it makes default_policy cast Resistance every round instead of attacking or leveling buffs (for Tammara it would preempt Divine Favor and never kill anything). A buff-priority policy is a redesign; no mechanical effect is applied."
+    {
+      "type": "buff",
+      "modifiers": [
+        { "target": "fort", "value": 1, "bonus_type": "resistance" },
+        { "target": "ref", "value": 1, "bonus_type": "resistance" },
+        { "target": "will", "value": 1, "bonus_type": "resistance" }
+      ],
+      "duration_rounds": 10
+    }
+  ],
+  "description": "SRD spells/resistance.md: a cantrip - the subject gains a +1 resistance bonus on saving throws for 1 minute. Will negates (harmless). Modeled as a BuffEffect granting +1 resistance to fort/ref/will for 10 rounds (1 minute); the resistance bonus type does not stack with itself, so a second Resistance (or a cloak of resistance) keeps the higher value. default_policy never casts it: cantrips have no spell slot, and _find_buff_spell skips spells the caster has no slot for, so Resistance cannot preempt a leveled buff (Divine Favor) in the known-spell order."
 }

src/pf1e_simulator/combat.py

@@ -8837,11 +8837,26 @@ def _find_offensive_spell(
     return None
 
 
-def _find_buff_spell(engine: CombatEngine, spells_known: list[str]) -> str | None:
+def _find_buff_spell(
-    """Find a buff spell."""
+    engine: CombatEngine, spells_known: list[str]
+) -> str | None:
+    """Find a buff spell the policy will actually cast.
+
+    PF1e cantrips (level 0) have no spell slot in the engine's budget
+    (``spell_slots_left`` has no level-0 key) and the policy deliberately
+    never auto-casts them (a cantrip buff would be cast every round, degrading
+    the policy), so a cantrip buff such as Resistance must not preempt a
+    leveled buff (Divine Favor) in the known-spell order. Leveled buffs keep
+    the historical first-match behavior: if the first one's slots are
+    exhausted the buff branch gives up (the golden pins that).
+    """
     for name in spells_known:
         spell = engine.spell_registry.get(name)
-        if spell and any(isinstance(e, BuffEffect) for e in spell.effects):
+        if (
+            spell
+            and spell.level > 0
+            and any(isinstance(e, BuffEffect) for e in spell.effects)
+        ):
             return name
     return None
 

tests/test_resistance.py

@@ -0,0 +1,249 @@
+"""Tests for the Resistance cantrip buff (SRD spells/resistance.md).
+
+Given: the Resistance cantrip encoding a +1 resistance bonus on fort/ref/will
+saves for 1 minute (10 rounds) as a BuffEffect
+When: casting on a willing ally / resolving saves / ticking durations / the
+default policy choosing a buff spell
+Then: the buff is applied without a save roll on a willing target, the
+resistance bonus type does not stack with itself, the bonus actually rides
+save rolls, and default_policy never proposes the cantrip (no spell slot) and
+never lets it preempt a leveled buff (Divine Favor) in the known-spell order.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+from pf1e_simulator.combat import (
+    CombatantState,
+    CombatEngine,
+    _choose_spell_action,
+    _find_buff_spell,
+)
+from pf1e_simulator.dice import parse_dice
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.models import (
+    AbilityScores,
+    ACProfile,
+    AttackSpec,
+    Combatant,
+    DamageComponent,
+    Saves,
+)
+from pf1e_simulator.rng import ScriptedRng
+from pf1e_simulator.spells import BuffEffect, load_spell_registry
+
+if TYPE_CHECKING:
+    from pf1e_simulator.effects import StatModifier
+
+_SPELLS_DIR = Path(__file__).resolve().parent.parent / "data" / "spells"
+
+
+def _make_combatant(
+    cid: str, *, casting: bool = False, spells: list[str] | None = None
+) -> Combatant:
+    attack = AttackSpec(
+        id=f"{cid}-w",
+        name="club",
+        kind="melee",
+        attack_bonus=3,
+        damage=[DamageComponent(formula=parse_dice("1d6"), types=["bludgeoning"])],
+    )
+    return Combatant(
+        id=cid,
+        name=cid,
+        level=4,
+        size="Medium",
+        abilities=AbilityScores(
+            str_score=10, dex_score=10, con_score=10,
+            int_score=18, wis_score=10, cha_score=10,
+        ),
+        hp_max=30,
+        ac=ACProfile(total=12, touch=12, flat_footed=12),
+        bab=2,
+        initiative_mod=0,
+        speed_land_ft=30,
+        saves=Saves(fort=2, ref=2, will=2),
+        attacks=[attack],
+        casting_ability="int" if casting else "wis",
+        spell_slots={1: 3} if casting else {},
+        spells=spells or [],
+    )
+
+
+def _make_engine(
+    queue: list[int],
+    *,
+    sides: dict[str, str] | None = None,
+    positions: dict[str, tuple[int, int]] | None = None,
+    combatants: list[Combatant] | None = None,
+) -> CombatEngine:
+    roster = combatants or [_make_combatant("hero")]
+    side_map = sides or {c.id: "players" for c in roster}
+    pos_map = positions or {c.id: (i, 0) for i, c in enumerate(roster)}
+    states = [
+        CombatantState(
+            combatant=c, side=side_map[c.id], pos=pos_map[c.id], hp=c.hp_max
+        )
+        for c in roster
+    ]
+    legend = {".": TerrainType(type="floor", move_cost=1)}
+    spec = MapSpec(name="test", terrain=tuple(["." * 12] * 12), legend=legend)
+    grid = Grid.from_spec(spec)
+    reg = load_spell_registry(_SPELLS_DIR)
+    return CombatEngine(ScriptedRng(queue), grid, states, spell_registry=reg)
+
+
+def _resistance_mods(state: CombatantState) -> list[StatModifier]:
+    return [m for b in state.active_buffs for m in b.modifiers]
+
+
+class TestResistanceLoading:
+    """Given: data/spells/resistance.json
+    When: load_spell_registry is called
+    Then: Resistance is a BuffEffect granting +1 resistance to fort/ref/will
+    for 10 rounds (1 minute)."""
+
+    def test_resistance_is_a_buff_effect(self) -> None:
+        reg = load_spell_registry(_SPELLS_DIR)
+        spell = reg["Resistance"]
+        assert len(spell.effects) == 1
+        effect = spell.effects[0]
+        assert isinstance(effect, BuffEffect)
+        assert effect.duration_rounds == 10
+        assert len(effect.modifiers) == 3
+        targets = {m.target for m in effect.modifiers}
+        assert targets == {"fort", "ref", "will"}
+        assert all(m.value == 1 for m in effect.modifiers)
+        assert all(m.bonus_type == "resistance" for m in effect.modifiers)
+
+
+class TestResistanceCasting:
+    """Given: a caster casting Resistance on a willing ally
+    When: _cast_spell is called
+    Then: the buff is applied without a save roll (harmless, willing target)
+    and the modifiers ride save rolls."""
+
+    def test_applies_buff_without_save_roll(self) -> None:
+        caster = _make_combatant("caster", casting=True)
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [3],
+            sides={"caster": "players", "target": "players"},
+            positions={"caster": (0, 0), "target": (1, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        engine._cast_spell(c, t, "Resistance")
+        mods = _resistance_mods(t)
+        assert len(mods) == 3
+        assert {m.target for m in mods} == {"fort", "ref", "will"}
+        assert all(m.value == 1 and m.bonus_type == "resistance" for m in mods)
+        # Will negates (harmless): a willing same-side target forgoes the save,
+        # so the cast consumes no d20 from the queue.
+        assert len(engine._rng._queue) == 1
+
+    def test_bonus_rides_save_rolls(self) -> None:
+        caster = _make_combatant("caster", casting=True)
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [10],
+            sides={"caster": "players", "target": "players"},
+            positions={"caster": (0, 0), "target": (1, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        engine._cast_spell(c, t, "Resistance")
+        # d20=10 + base will 2 + resistance 1 = 13 vs DC 12 (success).
+        result = engine.resolve_save(t, "will", 12)
+        assert result.success
+        assert result.total == 13
+
+    def test_resistance_does_not_stack_with_itself(self) -> None:
+        caster = _make_combatant("caster", casting=True)
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [10],
+            sides={"caster": "players", "target": "players"},
+            positions={"caster": (0, 0), "target": (1, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        engine._cast_spell(c, t, "Resistance")
+        engine._cast_spell(c, t, "Resistance")
+        # Two castings are two ActiveBuff entries, but the resistance bonus
+        # type does not stack: resolve_modifiers keeps the higher value (+1).
+        assert len(t.active_buffs) == 2
+        will_mods = [m for m in _resistance_mods(t) if m.target == "will"]
+        assert len(will_mods) == 2
+        result = engine.resolve_save(t, "will", 12)
+        assert result.total == 2 + 10 + 1  # base + d20 + single +1
+
+    def test_buff_expires_after_ten_rounds(self) -> None:
+        caster = _make_combatant("caster", casting=True)
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [3],
+            sides={"caster": "players", "target": "players"},
+            positions={"caster": (0, 0), "target": (1, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        engine._cast_spell(c, t, "Resistance")
+        assert len(t.active_buffs) == 1
+        for _ in range(10):
+            engine._decrement_durations(t)
+        assert t.active_buffs == []
+
+
+class TestResistancePolicy:
+    """Given: the default buff-spell selection
+    When: a caster knows both Resistance (cantrip) and Divine Favor (level 1)
+    Then: _find_buff_spell skips the slot-less cantrip and returns the leveled
+    buff, and _choose_spell_action proposes Divine Favor, never Resistance."""
+
+    def test_find_buff_spell_skips_slotless_cantrip(self) -> None:
+        caster = _make_combatant(
+            "caster", casting=True, spells=["Resistance", "Divine Favor"]
+        )
+        engine = _make_engine([3], combatants=[caster])
+        c = engine.states[0]
+        assert _find_buff_spell(engine, c.combatant.spells) == "Divine Favor"
+
+    def test_find_buff_spell_returns_none_without_leveled_buff(self) -> None:
+        caster = _make_combatant("caster", casting=True, spells=["Resistance"])
+        engine = _make_engine([3], combatants=[caster])
+        c = engine.states[0]
+        assert _find_buff_spell(engine, c.combatant.spells) is None
+
+    def test_choose_spell_action_proposes_divine_favor(self) -> None:
+        caster = _make_combatant(
+            "caster", casting=True, spells=["Resistance", "Divine Favor"]
+        )
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [3],
+            sides={"caster": "players", "target": "monsters"},
+            positions={"caster": (0, 0), "target": (5, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        actions = _choose_spell_action(engine, c, t)
+        assert actions is not None
+        assert actions[0].kind == "cast_spell"
+        assert actions[0].spell_name == "Divine Favor"
+        assert actions[0].target_id == c.combatant.id
+
+    def test_choose_spell_action_never_proposes_resistance(self) -> None:
+        caster = _make_combatant("caster", casting=True, spells=["Resistance"])
+        target = _make_combatant("target")
+        engine = _make_engine(
+            [3],
+            sides={"caster": "players", "target": "monsters"},
+            positions={"caster": (0, 0), "target": (5, 0)},
+            combatants=[caster, target],
+        )
+        c, t = engine.states
+        assert _choose_spell_action(engine, c, t) is None